Bad PCM??

[Heavyopp]

Hello, new to the forum -- Need some help with a troubled XJ

2000 Cherokee 4.0 -- 225,000 miles

Jeep starts but misses terribly -- Barely runs at idle -- runs when on throttle but very rough, misfires badly

When I bought it it needed the dual cats at the manifold -- cali emissions

bought the walker setup, re-welded all the joints, and installed all new O2 sensors because I didn't have 2 of them

So now that it's quiet The jeep is misfiring badly -- Changed plugs to champions, tested Ignition coil pack -- All Coils OHM out equally -- then tested spark with a steel brake line connected to ground -- All coils firing strong

After plugs I started getting codes -- One was injectors -- open circuit cylinders 2 and 4 -- I tested the wiring with no real results -- could never get a reading on my Volt meter -- I changed injectors -- upgraded to the ford mustang ones

Now with new injectors the injector codes are gone -- New code -- P0122 -- Throttle Position Sensor Low voltage

In an attempt to diagnose the TPS -- The 2 outside terminals on the plug 5 volts in and ground -- the center terminal on the plug is supposed to be TPS signal out -- I get 4.32 volts there with the TPS unplugged from the wiring harness

4.32 volts -- Only hooked up the the PCM -- Now I search wiring diagrams and find which pin is connected to TPS input on the PCM -- Pin #23 -- With the PCM wiring harness unplugged I get 6 volts on PIN # 23 -- I also get 7 volts on the pin which is supposed to be 5 volts to power the TPS -- I forget the pin number

So now what? -- Is my PCM fried? I don't even know where to look now...


Thanks for your thoughts -- hope you can understand this

Jer

[4.3LXJ]

I would check a couple of other sensors for input. It may be that you have fried it if it is putting out more than 5V

[Heavyopp]

It's funny -- With the plug disconnected at the PCM I get 7 volts at the pin that is supposed to supply the TPS with 5 volts -- When I plug the harness in I get 5 volts at the TPS as I should

The big problem is I'm getting voltage from the PCM at the terminal thats meant for TPS input

I would think that, with the TPS unplugged, I should get 0 volts at the terminal thats supposed to read the voltage from the TPS -- It's unplugged -- why is there voltage there?

[4.3LXJ]

I don't know. Sounds like you are getting voltage from something that shouldn't be giving it

[Heavyopp]

correct -- and it sure seems like it's coming directly from the #23 pin on the PCM

I guess it's as simple as that -- PCM fried

I certainly hold out for some thoughts from others for a few days -- The XJ is a play toy -- in no rush

[4.3LXJ]

It happens, they can go south on you

[Heavyopp]

SO I pulled my PCM -- took it apart -- No visible damage or burnt parts -- I'll do some testing tomorrow


Can someone test their TPS connector for me on a running XJ?

Pull the connector from your TPS -- with a voltmeter test the center terminal on the connector with the key on -- Positive probe on connector -- negative probe to a good ground somewhere ---Tell me if you have voltage there with the wires disconnected from the TPS



Thanks, Jer